In recent years, in the context of efficient use of energy, there have been demands mainly from transformer manufacturers and the like for an electrical steel sheet with high flux density, low iron loss, and low noise.
The flux density can be improved by making crystal orientations of the electrical steel sheet in accord with the Goss orientation. JP 4123679 B2 (PTL 1), for example, discloses a method for producing a grain-oriented electrical steel sheet having a flux density B8 exceeding 1.97 T.
On the other hand, iron loss properties may be improved by increased purity of the material, high orientation, reduced sheet thickness, addition of Si and Al, and magnetic domain refining (for example, see “Recent progress in soft magnetic steels,” 155th/156th Nishiyama Memorial Technical Seminar, The Iron and Steel Institute of Japan, Feb. 1, 1995 (NPL 1)). Additionally, JP 3386727 B2 (PTL 2) discloses a method for producing a grain-oriented steel sheet having a reduced coercive force by adjusting an annealing separator and exhibiting advantageous iron loss properties.
In addition, noise may be reduced by reducing the area of regions, called “closure domains,” with magnetic moment being oriented perpendicular to the external magnetic field direction. Many studies have reported on methods for reducing such closure domains, such as disclosed in JP 4585101 B2 (PTL 3), with particular consideration given to, among other things, the effectiveness of “according the <100> orientation of crystal grains with a rolling direction of the steel sheet” for improving flux density B8 and reducing hysteresis loss.
On the other hand, however, it is known that when the <100> orientation of crystal grains is in accord with the rolling direction, the magnetostatic energy decreases, and therefore the magnetic domain width widens, causing an increase in eddy current loss.
Therefore, as a method for reducing eddy current loss, some techniques have been used for refining magnetic domains by improving film tension and applying thermal strain.
Methods for improving film tension, such as disclosed in JP H02-8027 B2 (PTL 4), are effective for eliminating closure domains and thus advantageous for reducing noise. There are limits, however, on the amount of tension that can be applied to the steel sheet.
On the other hand, magnetic domain refining by applying thermal strain is performed by means of laser irradiation, electron beam irradiation and the like, and has a significant effect on reducing eddy current loss.
For example, JP H07-65106 B2 (PTL 5) discloses a method for producing an electrical steel sheet having a reduced iron loss W17/50 of below 0.8 W/kg by using electron beam irradiation. It can be seen from PTL 5 that the electron beam irradiation is extremely useful for reducing iron loss.
In addition, JP H03-13293 B2 (PTL 6) discloses a method for reducing iron loss by applying laser irradiation to the steel sheet.